Recently, to cope with high-quality and ultrabroadband sources such as DVD audio or super audio CDs, speakers capable of reproducing the sound up to ultrahigh frequencies of more than or equal to 100 KHz have been requested. There is also a need for high-pitched speakers capable of reproducing the sound up to ultrahigh frequencies at low cost, without regard to being a single components or small-sized stereo.
Conventionally, a high-pitched speaker in which a vibration diaphragm is driven using a piezoelectric element is suggested. However, since an acoustic generator using a piezoelectric element generally uses a resonance phenomenon, it is known that large peak dips occurred in frequency characteristics of the sound pressure and it is difficult to achieve satisfactory sound pressures up to ultrahigh frequencies.
Therefore, as a method for improving the peak dips in frequency characteristics in an acoustic generator using a piezoelectric element as a drive source, an acoustic generator disclosed in Patent Literature 1 is known.
The acoustic generator disclosed in Patent Literature 1 includes two disk-like piezoelectric elements disposed in two circular metal bases, respectively, and a single vibration diaphragm disposed to cover the two piezoelectric elements with a predetermined gap from the piezoelectric elements. The vibration diaphragm has a rectangular shape in a plan view which is convex in a direction in which sound is emitted. In such an acoustic generator, it is described that a high sound pressure is achieved up to about 100 KHz.
For example, according to Non Patent Literature 1, it is proven that the sound of ultrahigh frequency components of more than 20 KHz activates the human brain stem to have a good influence on a human being, such as an improvement in immune activity, a decrease in stress hormones, an enhancement of a waves in the brain, and making the sound of an audible frequency band of 20 KHz or lower more audible. The importance of an ultrahigh-frequency sound is becoming higher.